In one conventional image-forming apparatus provided with a belt unit that is detachably mountable in a main body of the image-forming apparatus, the belt unit includes a belt, and a drive roller and a follow roller over which the belt is mounted in a taut state (for example, please refer to Japanese Patent Application Publication No. 2011-075618). The main body of this image-forming apparatus is provided with a position-fixing part for fixing the position of the belt unit, and a belt drive gear that inputs a drive force into a drive roller gear provided on the drive roller. This drive force serves to press the belt unit against the position-fixing part.
However, changing the distance between the axes of the belt drive gear and the drive roller gear tends to change the conveying speed of the belt. In order to suppress such fluctuations in the conveying speed of the belt, a construction employing a coupling was conceived, whereby the coupling engages the drive roller in its axial direction and inputs a drive force into the drive roller. However, since the drive force inputted by the coupling does not generate a force for pressing the belt unit against the position-fixing part, as does the drive force of the belt drive gear, any disturbance or the like that changes the vertical position of the belt unit could cause the behavior of the belt to become unstable.